Iron alloy having anti-warping characteristics



Unimd States atent IRON ALLOY HAVING ANTI-WARPING CHARACTERISTICS Fred C. Woody, Garfield Heights, Ohio, assignor to National Malleable and Steel Castings Company, Cleveland, Ohio, a corporation. of Ohio No Drawing. Application August 1, 1956 Serial No. 601,380

4 Claims. (Cl. 148-3) This. invention relates to iron alloy castings of the type that are exposed to continual alternate heating and cooling, as for example the grate bars used in sintering machines, and more particularly to an improved chemical composition and subsequent heat treatment procedure for such castings which will appreciably increase their resistance to warp-age and wear.

In sintering machines of the usual type, there is an endless conveyor made up of a series of pallets on which grate bars are mounted. The grate bars form a supporting surface of the pallet and receive thereon material to be sintered. It has been found that in service, the outer ends of these grate bars have a tendency to warp upwardly, due mostly to the repeated heating and cooling of the bars during sintering operations. This warpage of the bars is undesirable for various reasons, one being the resulting uneven surface of the grate bed. In addition, the warped outer ends of the bars sometimes become so positioned that they overlap adjacent bars, thus leaving gaps in the grate bed adjacent its outer edges. The lateral edges of the gnate bars adjacent these gaps burn out and deteriorate much faster than the remaining portions of the bars, due to the increased passage of hot gases through these openings and also due to the abrasive wear caused by the sintered material dropping through these gaps. Consequently, the bars do not wear uniformly throughout their full length and have to be replaced prematurely. Moreover, the material dropping through the gaps in the grates is lost in the sintering process and creates an untidy state in the area surrounding the sintering machine. Furthermore, the sintered material lost from the grate causes increased wear on the circulating fan and duct system of the machine.

It is, therefore, a general object of the invention to provide a new and improved chemical composition for castings, especially grate bar castings, which will result in an article of manufacture having a high degree of resistance to warpage and abrasive wear.

Another object of the invention is to provide an iron alloy casting, having :a new and improved composition which, when subjected to a particular heat treating procedure results in a casting having appreciable resistance to warpage.

A more specific object of the invention is to provide a white cast iron composition for grate bars especially that type of bar subjected to alternate heating and cooling having a high manganese content of the order of 1.5 percent to 2.50 percent by weight, and which results in a casting having improved resistance to warpage and increased Wear life.

Other objects and advantages of the invention will be apparent from the following description.

An approximate chemical analysis of prior art iron ice alloy compositions often used for grate bars and other similar purposes is as follows:

, Percent Carbon 2.0 to 3.5 Silcon 0.5 to- 3.0 Manganese 0.18 to 0.80 Sulphur up to 0.2 Phosphorus 0.05 to 0.15

The balance of the metal being substantially all iron.

While this produces an iron alloy which has some duetility and is useful for a great many purposes, when used for grate bars and other similar articles which are.

subjected to alternate heating and cooling, it has in.- adequate resistance to warpage and abrasive wear.

The Warpage, in service, of existing grate bars having approximately the above chemical composition is due principally to the following three causes:

(1) The uneven heating and cooling of the bars- (2) Growth of the internal structure of the bars because of graphitization.

(3) Growth of the bars because of internal oxidation.

The uneven, comparatively rapid heating and cooling of the bars during the sintering process causes uneven expansion and contraction of the bars which eventually causes the latter to deform by warping.

The second reason listed above for warping growth caused by graphitization is also primarily a result of the heating and cooling of the bars during the sintering process. As the bars cool from their heated condition during that portion of the sintering cycle when the pallets have discharged their loads and are travelling back to the feeding end of the sintering machine, substantial graphitization occurs, resulting in. the precipitation of free carbon throughout the bar structure. The free carbon tends to coalesce and by acquiring greater volume, promotes unequal or unbalanced growth and subsequent Warpage of the bars.

As graphitization continues over a period of time, considerable internal stresses occur, especially about the free particles of carbon, which give rise to internal fissures and cavities in the structure of the bars. This makes the bars comparatively porous: and thus subject to oxida tion by the surrounding air. Oxygen from the atmosphere enters the fissures and voids in the bars, resulting in accelerated oxidation as the air unites chemically with the iron to form iron oxide. The oxide formed being of greater volume than the original cavity, promotes additional growth and warpage of the bars.-

It has been discovered that by increasing the manganese content of the castings to within a range of 1.5 percent to 2.50 percent, an article is produced which has a high degree of resistance to warpage and abrasive wear, even though it is subjected to repeated heating and cooling. Such a casting lends itself ideally for such purposes as grate bars for sintering machines.

In accordance with the invention, a composition for 1 g 3 producing a casting having such anti-warping and longlife wear characteristics is approximately as follows:

"It .will be understood that while the above listed limits in the percentages of sulphur and phosphorus are desirable, the amounts of such elements in the preferred composition, in actual practice, will usually be controlled by the amounts of such elements already existing in the materials used to make up the composition. It is particularly desirable to maintain the phosphorus content below the above noted maximum (preferably around .07%) in order to remove any tendency of brittleness in the finished product.

The high percentage of manganese results in a casting which is comparatively rigid and has relatively high strength characteristics and abrasive resistance. Accordingly, the uneven heating and cooling of such a casting, as occurs in grate bars used in sintering machines, will not result in the degree of warping inherent in prior art compositions because of the greater physical rigidity and strength of the high manganese bar. Moreover, the ex cess manganese effectively retards graphitization in the casting, which thereby reduces or eliminates the growth tendency and associated warpage. In this connection, it will be understood that the manganese retards graphitization not only during the heating and cooling cycles to which the casting is subjected in service, but also retards graphitization during the original pouring operation during which the molten metal solidifies in the mold to form the casting. As the metal cools and solidifies, the excess manganese retards the separation of the carbon from the usual cementite and pearlite structure of the casting to thus limit the particles or nuclei of carbon interspersed throughout the final casting. Thus, during subsequent heating and cooling of the casting in service, the carbon precipitating out will again be limited by the excess manganese and any that does occur will tend to migrate to the reduced number of previously established nuclei, thereby to reduce growth and associated warpage of the article. Since graphitization throughout the structure is greatly reduced, very little, if any, internal stresses are set up adjacent the particles of free carbon, thus eliminating the tendency of the bars to become porous. This, of course, prevents internal oxidation of the bars and eliminates one of the principal causes of warping of present grate bars.

Another aspect of the invention is the heat treatment given to the castings composed of the above noted high manganese composition. During the original pouring and cooling of the castings some internal stresses naturally develop. In order to relieve such stresses the solidified castings are subjected to a heat treatment which consists of reheating the castings to just slightly above the critical temperature range (or generally to approximately 1500 F.) and then allowing them to cool slowly to room temperature. This procedure results in a considerable saving in time and expense over the prior art procedures described above that necessitate heating the castings for long periods of time at comparatively high temperatures. In this connection, the present procedure eliminates the need of cleaning any scale ofi the castings after the heat treatment, since the castings are not heated to a sutficient temperature or maintained at that temperature sufiiciently long to produce any oxidation or scaling of the outer surfaces of the casting. This elimination of the step of cleaning in the production of the castings results in a. substantial saving in material and labor.

From the foregoing description it will be readily understood that there is provided a new and improved high manganese composition for iron alloy castings and especially grate bar castings. Such composition results in an article of manufacture, such as a grate bar, having improved resistance to warpage and abrasion, despite the continual heating and cooling cycles to which such articles are subjected in service.

It will also be understood that the invention provides a novel heat treating process that is simple and much less expensive than the processes utilized in heat treating present types of grate bars and eliminates the need of cleaning the bars of scale after the heat treating process is completed.

The terms and expressions which have been employed are used as terms of description and not of limitation and there is no intention in the use of such terms and expressions of excluding any equivalents of the features described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.

What is claimed is:

1. A method for heat treating a grate bar for a sintering machine containing 2.5% carbon, 1.25% silicon, and 1.5% to 2.50% manganese, the balance substantially all iron and incidental impurities, comprising casting the bar and then heating the bar, to just above the critical temperature of the metal, discontinuing the heat after reaching the said temperature, and then allowing the bar to cool slowly to room temperature.

2. A method of making grate bar castings for sintering machines comprising the steps of preparing a cast iron melt having substantially a composition of 2.5% carbon, 1.25 silicon, .175 sulphur, .07% phosphorus and 1.5% to 2.50% manganese, the balance being substantially all iron, pouring the molten metal into a mold and allowing the metal to cool slowly in the mold to room temperature, removing the casting from the mold and reheating the same to a temperature just above the critical temperature of the metal, and then permitting the reheated casting to cool to room temperature.

3. A method in accordance with claim 2, wherein the casting is reheated to approximately 1500 F. after solidification and removal thereof from the mold.

4. A grate bar of white cast iron for sintering machines containing the normal alloying elements in percentages by weight of about 2.5% carbon, about 1.25% silicon, the normal impurities of sulphur and phosphorus, and a high manganese content of about 1.5% to 2.50%, the manganese being operative to inhibit carbon precipitation during both the original solidification of the casting and during subsequent continual heating and cooling cycles in service, said bar being given a stress relieving heat treatment prior to being placed in service comprising reheating of the bar after solidification to a temperature just above the critical temperature range of the metal, discontinuing the heat after reaching the said temperature and then allowing the bar to cool slowly to room temperature.

References Cited in the file of this patent UNITED STATES PATENTS 1,166,342 Field at al Dec. 28, 1915 7 1,211,826 Daniels Jan. 9, 1917 2,219,320 Jones Oct. 29, 1940 FOREIGN PATENTS 740,570 France Jan. 27, 1933 

2. A METHOD OF MAKING GRATE BAR CASTINGS FOR SINTERING MACHINES COMPRISING THE STEPS OF PREPARING A CAST IRON MELT HAVING SUBSTANTIALLY A COPPOSITON OF 2.5% CARBON, 1.25% SILICON, .175% SULPHUR, .07% PHOSPHORUS AND 1.5% TO 2.50% MANGANESE, THE BALANCE BEING SUBSTANTIALLY ALL IRON, POURING THE MOLTEN METAL INTO A MOLD AND ALLOWING THE METAL TO COOL SLOWLY IN THE MOLD TO ROOM TEMPERATURE, REMOVING THE CASTING FROM THE MOLD AND REHEATING THE SAME TO A TEMPERATURE JUST ABOVE THE CRITICAL TEMPERATURE OF THE METAL, AND THEN PERMITTING THE REHEATED CASTING TO COOL TO ROOM TEMPERATURE. 